Low voltage system for audio amplifiers

ABSTRACT

In some embodiments, a low voltage system can include a capacitor between an output node of an amplifier and ground, with the output node connectable to a load, and the amplifier configured to operate with a series of pulses. The low voltage system can further include a monitoring circuit configured to monitor a voltage at the capacitor against a desired low voltage value, and a control system configured to generate the series of pulses for the amplifier, and to control charging and discharging of the capacitor based on an output of the monitoring circuit to regulate the voltage at the output node at approximately the desired low voltage value.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application No.63/250,185 filed Sep. 29, 2021, entitled LOW VOLTAGE SYSTEM, thedisclosure of each of which is hereby expressly incorporated byreference herein in its respective entirety.

BACKGROUND Field

The present disclosure relates to audio amplifier circuits for wearableaudio devices such as earbuds or headphones.

Description of the Related Art

A wearable audio device can be worn by a user to allow the user to enjoylistening of an audio content stream being played by a mobile device.Such an audio content stream may be provided from the mobile device tothe wearable audio device through, for example, a short-range wirelesslink. Once received by the wearable audio device, the audio contentstream can be processed by one or more circuits to generate an outputthat drives a speaker to generate sound waves representative of theaudio content stream.

It is desirable to have the foregoing sound waves provide reproductionof the audio content stream with high-fidelity. It is also desirable forthe wearable audio device to operate in a power-efficient manner, sincesuch devices are commonly powered by batteries having limitedcapacities.

SUMMARY

In some implementations, the present disclosure relates to a low voltagesystem that includes a capacitor between an output node of an amplifierand ground, with the output node connectable to a load, and theamplifier configured to operate with a series of pulses. The low voltagesystem further includes a monitoring circuit configured to monitor avoltage at the capacitor against a desired low voltage value, and acontrol system configured to generate the series of pulses for theamplifier, and to control charging and discharging of the capacitorbased on an output of the monitoring circuit to regulate the voltage atthe output node at approximately the desired low voltage value.

In some embodiments, the amplifier can be an audio amplifier such as aClass D audio amplifier. In some embodiments, the audio amplifier can beconfigured to operate as a pulse-width modulation (PWM) amplifier.

In some embodiments, some or all of an audio amplification system havingthe foregoing low voltage system can be implemented on a semiconductordie, a packaged module, or some combination thereof.

In some embodiments, some or all of an audio amplification system havingthe foregoing low voltage system can be implemented in a wireless devicesuch as a wireless headphone or a wireless earphone.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system that includes a wearable audio device incommunication with a host device, where the wearable audio deviceincludes an audio amplifier circuit.

FIG. 2 shows that the wearable audio device of FIG. 1 can be implementedas a device configured to be worn at least partially in an ear canal ofa user.

FIG. 3 shows that the wearable audio device of FIG. 1 can be implementedas part of a headphone configured to be worn on the head of a user, suchthat the audio device is positioned on or over a corresponding ear ofthe user.

FIG. 4 shows that in some embodiments, the audio amplifier circuit ofFIG. 1 can include a number of functional blocks.

FIG. 5 shows a block diagram of an audio amplifier circuit that is amore specific example of the audio amplifier circuit of FIG. 4 .

FIG. 6 shows a block diagram that includes a digital audio path blockthat is a more specific example of the digital audio path block of FIG.5 .

FIG. 7 shows a block diagram of an amplifier block that is a morespecific example of the amplifier block of FIG. 5 .

FIG. 8 shows a block diagram of a digital logic circuit block that is amore specific example of the digital logic circuit block of FIG. 5 .

FIG. 9 shows a gain compute block that is a more specific example of thegain compute block of FIG. 8 .

FIG. 10 shows various functional blocks of the audio amplifier circuitof FIG. 5 configured to provide a gain calibration sub-system.

FIG. 11 shows a more detailed example of the gain calibration block ofFIG. 10 .

FIG. 12 shows a circuit diagram of a low voltage system that can beutilized in an audio amplifier having one or more features as describedherein.

FIG. 13 shows the low voltage system of FIG. 12 being operated withspecific example settings.

FIG. 14 shows examples of various signal traces during the operation ofthe low voltage system of FIG. 13 .

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

FIG. 1 depicts a system 1010 that includes a wearable audio device 1002in communication with a host device 1008. Such communication, depictedas 1007 in FIG. 1 , can be supported by, for example, a wireless linksuch as a short-range wireless link in accordance with a common industrystandard, a standard specific for the system 1010, or some combinationthereof. In some embodiments, the wireless link 1007 includes digitalformat of information being transferred from one device to the other(e.g., from the host device 1008 to the wearable audio device 1002).

In FIG. 1 , the wearable device 1002 is shown to include an audioamplifier circuit 1000 that provides an electrical audio signal to aspeaker 1004 based on a digital signal received from the host device1008. Such an electrical audio signal can drive the speaker 1004 andgenerate sound representative of a content provided in the digitalsignal, for a user wearing the wearable device 1002.

In FIG. 1 , the wearable device 1002 is a wireless device; and thustypically includes its own power supply 1006 including a battery. Such apower supply can be configured to provide electrical power for the audiodevice 1002, including power for operation of the audio amplifiercircuit 1000. It is noted that since many wearable audio devices havesmall sizes for user-convenience, such small sizes places constraints onpower capacity provided by batteries within the wearable audio devices.

In some embodiments, the host device 1008 can be a portable wirelessdevice such as, for example, a smartphone, a tablet, an audio player,etc. It will be understood that such a portable wireless device may ormay not include phone functionality such as cellular functionality. Insuch an example context of a portable wireless device being a hostdevice, FIGS. 2 and 3 show more specific examples of wearable audiodevices 1002 of FIG. 1 .

For example, FIG. 2 shows that the wearable audio device 1002 of FIG. 1can be implemented as a device (1002 a or 1002 b) configured to be wornat least partially in an ear canal of a user. Such a device, commonlyreferred to as an earbud, is typically desirable for the user due tocompact size and light weight.

In the example of FIG. 2 , a pair of earbuds (1002 a and 1002 b) can beprovided—one for each of the two ears of the user—and each earbud caninclude its own components (e.g., audio amplifier circuit, speaker andpower supply) described above in reference to FIG. 1 . In someembodiments, such a pair of earbuds can be operated to provide, forexample, stereo functionality for left (L) and right (R) ears.

In another example, FIG. 3 shows that the wearable audio device 1002 ofFIG. 1 can be implemented as part of a headphone 1003 configured to beworn on the head of a user, such that the audio device (1002 a or 1002b) is positioned on or over a corresponding ear of the user. Such aheadphone is typically desirable for the user due to audio performance.

In the example of FIG. 3 , a pair of audio devices (1002 a and 1002 b)can be provided—one for each of the two ears of the user. In someembodiments, each audio device (1002 a or 1002 b) can include its owncomponents (e.g., audio amplifier circuit, speaker and power supply)described above in reference to FIG. 1 . In some embodiments, one audiodevice (1002 a or 1002 b) can include an audio amplifier circuit thatprovides outputs for the speakers of both audio devices. In someembodiments, the pair of audio devices 1002 a, 1002 b of the headphone1003 can be operated to provide, for example, stereo functionality forleft (L) and right (R) ears.

FIG. 4 shows that in some embodiments, the audio amplifier circuit 1000of FIG. 1 can include a number of functional blocks. More particularly,in FIG. 4 , an audio amplifier circuit 1000 is shown to include adigital logic circuit block 1020, an amplifier block 1022, a powermanagement block 1024, and an ancillary block 1026. Examples related tosuch blocks are described herein in greater detail.

In FIG. 4 , the audio amplifier circuit 1000 is shown to further includevarious interfaces to allow the audio amplifier circuit 1000 to interactwith other devices external to the audio amplifier circuit 1000. Moreparticularly, an interface indicated as 1030 can be configured tosupport input/output (I/O) functionality with respect to a host device(e.g., 1008 in FIG. 1 ). An interface indicated as 1034 can beconfigured to support providing of electrical audio signals to a speaker(e.g., 1004 in FIG. 1 ). An interface indicated as 1032 can beconfigured to support providing of electrical power to various parts ofthe audio amplifier circuit 1000. One or more ground pins collectivelyindicated as 1036 (GND) can be configured to provide a groundingconnection for the audio amplifier circuit 1000 relative to, forexample, the audio device 1002 of FIG. 1 .

FIG. 5 shows a block diagram of an audio amplifier circuit 1000 that isa more specific example of the audio amplifier circuit 1000 of FIG. 4 .In FIG. 5 , a digital logic circuit block, generally indicated as 1020,can include a number of more specific functional blocks; an amplifierblock, generally indicated as 1022, can include a number of morespecific functional blocks; a power management block, generallyindicated as 1024, can include a number of more specific functionalblocks; and an ancillary block, generally indicated as 1026, can includea number of more specific functional blocks. Similarly, an interfaceindicated as 1030 can include a number of pins to support input/output(I/O) functionality with respect to a host device; an interfaceindicated as 1034 can include a number of pins to support providing ofelectrical audio signals to a speaker; an interface indicated as 1032can include a number of pins to support providing of electrical power tovarious parts of the audio amplifier circuit 1000; and one or moreground pins collectively indicated as 1036 (GND) can be implemented toprovide a grounding connection for the audio amplifier circuit 1000.

Referring to the example of FIG. 5 , the digital logic circuit block1020 can include a receiver (Rx) block 1040 configured to receive, forexample, a pulse-density modulation (PDM) signal through a DATA pin ofthe interface 1030. The PDM Rx block 1040 is shown to also receive aclock signal through a CLK pin of the interface 1030. The PDM Rx block1040 is shown to provide an output based on the input PDM signal.

It will be understood that while various examples are described hereinin the context of pulse-density modulation of signals, one or morefeatures of the present disclosure can also be implemented utilizingother types of modulations including other types of pulse modulations.

In FIG. 5 , the digital logic circuit block 1020 can further include adigital audio path block 1042. Such a block is shown to receive theoutput of the PDM Rx block 1040 and route the received signal to theamplifier block 1022. Additional examples related to the digital audiopath block 1042 are described herein in greater detail.

As shown in FIG. 5 , the digital logic circuit block 1020 can alsoinclude various blocks for providing control and calibrationfunctionalities. For example, amplifier controller 1090, resistancenetwork control block 1064, amplifier operating mode (e.g., HOR/ZORmode) control block 1062, inter-integrated circuit (I2C) auxiliary block1092, registers block 1094, PDM detect block 1060 and loudnessprotection block 1066 can provide and/or support various controlfunctionalities described herein. In another example, current ratiomeasurement calibration block 1068 and gain calibration block 1070 canprovide calibration functionalities described herein. More particularly,the current ratio measurement calibration block 1068 can supportgeneration of a reference signal for a loop circuit as described herein,and the gain calibration block 1070 can provide various functionalitiesfor gain calibration as described herein.

Referring to the example of FIG. 5 , the amplifier block 1022 is shownto include a pulse-width modulation (PWM) controller 1050 configured toreceive a feedforward digital signal from the digital audio path block1042 of the digital logic circuit block 1020 (through a path indicatedas 1043) and generate control signals for an H-bridge driver 1052. TheH-bridge driver 1052 provides analog electrical audio signals HPN, HPPas outputs. Such electrical audio signals can be provided to a speakerthrough respective pins of the interface 1034.

In the example of FIG. 5 , the amplifier block 1022 is configured as adigital PWM Class D amplifier. In addition to the H-bridge driver 1052being pulse-width modulated by the PWM controller 1050 based on thefeedforward digital signal from the digital audio path block 1042, aclosed-loop architecture is provided. Such a closed loop is shown toinclude an input resistance network 1080 coupled to the HPN and HPPoutputs of the H-bridge driver 1052, with the input resistance network1080 being coupled to a loop filter 1046 through summing nodes 1081,1083. An analog output from the loop filter 1046 is shown to beconverted into a digital signal by an analog-to-digital converter (ADC)1048 such as a successive approximation register (SAR) ADC. The digitalsignal from the SAR ADC 1048 is provided to the PWM controller 1050.

In the example of FIG. 5 , the amplifier block 1022 is configured toprovide a reference analog signal for the foregoing closed-loop circuit.More particularly, a digital signal from the digital audio path block1042 is shown to be provided to a digital-to-analog converter (DAC) 1044(through a path indicated as 1045), and the resulting analog signal isprovided to the summing nodes 1081, 1083. The summing nodes 1081, 1083are also shown to be provided with respective signals from acommon-mode-limit (CML) amplifier 1082.

In the example of FIG. 5 , the audio amplifier circuit 1000 is shown toinclude a gain calibration feature. Such a feature is shown to include acalibration ADC 1084 coupled to the HPN and HPP outputs of the H-bridgedriver 1052 to provide a digital signal representative of the analogoutput signals of the H-bridge driver 1052. The digital signal from thecalibration ADC 1084 is shown to be provided to the gain calibrationblock 1070 of the digital logic circuit block 1020.

In the example of FIG. 5 , the H-bridge driver 1052 shown to be providedwith multiple levels of supply voltages (e.g., VBAT, VDD_A, VDD_B,VDD_D, VDD_E). Such multiple voltage levels can allow the H-bridgedriver 1052 to operate with improved power efficiency.

Additional examples concerning the amplifier block 1022 are describedherein in greater detail.

In FIG. 5 , the power management block 1024 can include a number offunctional blocks configured to provide and/or support providing ofpower to various parts of the audio amplifier circuit 1000. For example,the power management block 1024 can be configured to provide routing ofmultiple supply voltage levels (e.g., VBAT, VDD_A, VDD_B, VDD_D, VDD_E)to the H-bridge driver 1052 of the amplifier block 1022. For the examplesupply voltage levels, VBAT>VDD_A>VDD_B>VDD_D>VDD_E.

Such supply voltages can be provided from source(s) external to theaudio amplifier circuit 1000, from internal source(s), or somecombination thereof. In the example of FIG. 5 , supply voltages VBAT,VDD_A and VDD_B are provided from external source(s); VDD_D may beprovided from an external source or from an internal source implementedas a low drop out (LDO) regulator 1130; and VDD_E is provided from aninternal source implemented as a low voltage monitor (LVM) supply 1132.

Some or all of the foregoing voltages can be monitored by one or morevoltage monitors. For example, a supply voltage monitor (SVM) 1120 isshown to monitor the voltages VBAT, VDD_A, VDD_B and VDD_D. Such an SVMcan include low power low resolution ADCs that monitor the supplyvoltages and produce respective digital outputs representative of thesupply voltages; and such monitored digital outputs can be utilized byother digital circuitry to control various functionalities of the audioamplifier circuit 1000. In another example, the voltage VDD_E is shownto be self-monitored by the LVM supply 1132.

Referring to the example of FIG. 5 , the power management block 1024 isshown to further include a reference (Ref) block 1110. Such a referenceblock can be implemented as a low voltage, low power bandgap referencecircuit configured to operate with a supply voltage (e.g., VDD_B) toproduce a low reference voltage as an output. Such a reference voltagecan be utilized for operation of an analog LDO regulator 1112 and adigital LDO regulator 1114, as well as other functional blocks of theaudio amplifier circuit 1000. The analog LDO regulator 1112 can beimplemented as a lower power linear regulator configured to provide adesired voltage for a number of circuits of the audio amplifier circuit1000. The digital LDO regulator 1114 can be implemented as a low powerlinear regulator configured to provide a desired voltage for variousdigital logic and digital core circuits of the audio amplifier circuit1000.

Referring to the example of FIG. 5 , the power management block 1024 isshown to further include a low power oscillator (LPO) 1118. Such an LPOcan be configured to support operation of the audio amplifier circuit1000. The power management block 1024 is shown to further include asensor block 1116 such as a temperature sensor. Such a sensor can beconfigured to detect operating condition(s) (e.g., temperature) of someor all of the audio amplifier circuit 1000; and such sensed condition(s)can be utilized to support one or more functionalities (e.g., faultprotection) for the audio amplifier circuit 1000.

In FIG. 5 , the ancillary block 1026 is shown to include apower-on-reset (POR) block 1100. Such a POR block can be implemented toprovide a number of functionalities. For example, power-on resetfunctionality can be provided by the POR block 1100, where the POR block1100 monitors the RESET_B pin and supply voltage conditions to controland/or support power-on sequencing of various regulators, clock systemand wall level shifters utilizing respective control signals (Pups).Once such power-on sequencing is achieved and the controlled componentsare operating in a stable manner, a release signal (Dig_reset_B) isprovided to allow operation of various digital blocks.

The POR block 1100 can also control and/or support a power-downsequence. Such a power-down sequence can be achieved in response to acontrol signal from a host device (e.g., setting RESET_B to a initiatepower-down), or based on detection of one or more conditions. Suchconditions can include, for example, a brownout detection and variousfault detections.

In FIG. 5 , the ancillary block 1026 is shown to include a one-timeprogrammable memory (OTP) 1102 and a block 1104 providing control andregister functionalities for the OTP block 1102. Such functionalitiescan include issuing of a control signal (I2C address) to the 120 block1092 to load appropriate registers of the Registers block 1094 during aboot process.

FIG. 6 shows a block diagram that includes a digital audio path block1042 that is a more specific example of the digital audio path block1042 of FIG. 5 . In FIG. 6 , a pulse-density modulation (PDM) receiver(Rx) block 1040 is shown to receive a PDM signal DATA and a clock signalCLK, and provide a PDM digital signal to the digital audio path block1042.

More particularly, the PDM digital signal from the PDM Rx block 1040 isshown to be provided to a digital low-pass filter (PDM LPF) 1140. Such afilter block can be configured to, for example, attenuate out-of-bandnoise in the received PDM digital signal (e.g., noise resulting from atransmit (Tx) modulation in a host device). The PDM LPF block 1140 ofFIG. 6 can also be configured to convert the PDM input signal into anoutput digital signal having pulse-code modulation (PCM).

In FIG. 6 , the filtered PCM signal from the PDM LPF block 1140 is shownto be provided to an equalizer (EQ) block 1142. Such an EQ block can beconfigured to support gain and mute functionalities, as well ashigh-output resistance (HOR) and/or zero-output resistance (ZOR)operating modes. The EQ block 1142 provides an output PCM signal to acalibration tone mixer 1144. Additional details concerning the EQ block1142 are provided herein.

Referring to FIG. 6 , the calibration tone mixer 1144 is shown to mix acalibration tone signal from a gain calibration block 1070 with theoutput PCM signal from the EQ block 1142. Such mixing functionality canbe provided during a calibration process such as an HOR/ZOR gaincalibration process. Additional details concerning the HOR/ZOR gaincalibration process are provided herein.

In FIG. 6 , the output of the calibration tone mixer 1144 is also shownto be processed through a number of blocks before being provided adigital-to-analog converter (DAC) 1044 (also 1044 in FIG. 5 ) through apath 1045 for a closed-loop circuit as described herein. Moreparticularly, the output (PCM signal) of the calibration tone mixer 1144is shown to be provided to a signal limiter block 1146. Such a signallimiter can be configured to maintain a threshold limit for a DACmodulator (e.g., delta sigma modulator (DSM)) 1148 and also improvehandling of low supply voltage operating conditions (e.g., when a HORmode load reactance causes supply voltage requirements of an H-bridgedrive to increase beyond a pure resistive load condition).

Referring to FIG. 6 , it is noted that for the foregoing low voltageoperating conditions, a SAR ADC (e.g., 1048 in FIG. 5 ) in theclosed-loop circuit can be driven into saturation if the power supply isinsufficient to provide the required peak voltage for the H-bridgedriver (1052 in FIG. 5 ) needed to develop an output voltage at theload. If such a SAR ADC is saturated, the corresponding loop filter(1046 in FIG. 5 ) is saturated and is slow to recover from such overloadconditions, resulting in undesirable audio artifacts. Thus, it isdesirable to have the SAR ADC and loop filter prevented from enteringsaturation. Such saturation-prevention can be accomplished by limitingthe digital audio signal in digital path of the closed-loop circuit toprevent the closed-loop circuit from trying to generate an unachievableoutput voltage and thereby push the SAR ADC and loop filter intosaturation.

Referring to FIG. 6 , the signal limiter 1146 can be configured toprevent the SAR ADC and loop filter from being in saturation due toinsufficient supply voltage while trying to produce an output voltagefrom the corresponding digital input signal. The signal limiter 1146 canlimit (e.g., by clipping the digital audio signal to a clip level) basedon either or both of loop filter saturation detection signal (LF Sar.Det.) and SAR ADC output level (SAR ADC out) to keep the SAR ADC and theloop filter from overloading. It is noted that the SAR ADC output levelcan be monitored to have the signal limiter to clip the digital audiosignal when the SAR ADC output level is close to saturation.

In FIG. 6 , the signal limiter block 1146 is shown to provide an outputsignal to a DAC modulator (e.g., delta sigma modulator (DSM)) 1148. Asdescribed above, such an output of the signal limiter block 1146 can beclipped to avoid the SAR ADC and loop filter from being in saturation.The output of the signal limiter block 1146 can also prevent the DAC DSM1148 from overloading.

In the example of FIG. 6 , the DAC DSM block 1148 can be configured tore-modulate a higher-bit input signal (e.g., 24-bit signal), throughdelta-sigma modulation, into a lower-bit signal (e.g., 9-bit signal)that is appropriate for a dynamic element matching (DEM) block 1150.Accordingly, the example 9-bit DSM output signal drives the DEM block1150 which can be implemented as a digital block configured to, forexample, randomize a pattern of 512-bit cell drive to the DAC 1044 in amanner to linearize the DAC's response for use as a multi-bitdelta-sigma DAC. Such a configuration can provide a desirable referenceaudio signal for the closed-loop circuit described herein.

In FIG. 6 , a feedforward signal is shown to be provided to a PWMcontroller (1050 in FIG. 5 ) from an output of the signal limiter 1146through a path 1043. As described herein, such a feedforward signal mayor may not include a calibration tone signal mixed therein, depending onoperating status of the gain calibration process.

In addition to the PDM LPF block 1140, EQ block 1142, mixer 1144, signallimiter block 1146, DSM block 1148 and DEM block 1150 that can begenerally referred to as the digital audio path block 1042, FIG. 6 alsoshows a number of blocks that support various functionalities of theaudio amplifier circuit (1000 in FIG. 5 ). For example, a PDM detectionblock 1060 (also 1060 in FIG. 5 ) is shown to be coupled to the PDM Rxblock 1040. The PDM detection block 1060 can be configured to detect oneor more states of PDM digital audio interface, including one or morefault conditions, to support operation and control of the audioamplifier circuit 1000.

In another example, a loudness protection block 1066 (also 1066 in FIG.5 ) is shown to be coupled to the PDM Rx block 1040. The loudnessprotection block 1066 can be configured to monitor both of two outputchannels (main channel and auxiliary channel) of the PDM Rx block 1040.Upon loudness detection (e.g., digital audio signal exceeding athreshold), the loudness protection block 1066 can issue a fault signalto provide a muting or fault condition functionality.

In FIG. 6 , the loudness protection block 1066 can include a pair offilter stages for the two output channels of the PDM Rx block 1040. Eachfilter stage can include a cascaded low-pass filter and high-pass filterstructure configured to approximate a frequency response such as anA-weighted frequency response. The filtered outputs can be provided torespective absolute value circuits, and outputs thereof can then beprovided to comparator circuits and compared against a programmedthreshold for each of the main channel, auxiliary channel andmain-minus-auxiliary values. Logic outputs of the comparators can besent to a multiplexer that can trigger a fault signal depending on acombination of the three comparator outputs.

In FIG. 6 , a number of functional blocks are shown to be coupled toand/or be related to one or more functional blocks of the digital audiopath block 1042. For example, a digital-to-analog converter (DAC) 1044is shown to be coupled to an output of the DEM block 1150. Such a DACcan be utilized to provide a reference signal for a closed-looparchitecture of the audio amplifier circuit (1000 in FIG. 5 ).Additional details concerning the closed-loop architecture are describedherein.

In another example, a HOR/ZOR state control block 1062 is shown to becoupled to the EQ block 1142. Such a control block, along with aresistance control (Rout ctrl) block 1064 and a resistance network 1080,can be utilized to provide various functionalities associated withhigh-output resistance (HOR) and zero-output resistance (ZOR) operatingmodes. Additional details concerning such operating modes are describedherein.

In yet another example, a gain calibration block 1070 is shown toprovide a calibration tone to the mixer 1144 based on inputs from signallimiter 1146 and a calibration ADC 1084. Additional details concerninggain calibration of the audio amplifier circuit (1000 in FIG. 5 ) aredescribed herein.

In FIG. 6 , operations of various functional blocks are shown to besupported by audio path registers 1094. Such registers can be a part ofor associated with the registers block 1094 of FIG. 5 and be configuredin a similar manner.

FIG. 7 shows a block diagram of an amplifier block 1022 that is a morespecific example of the amplifier block 1022 of FIG. 5 . The amplifierblock 1022 of FIG. 7 includes a digital PWM synthesis class D amplifierarchitecture. It is noted that unlike a purely analog class D amplifierarchitecture where pulse-width modulation (PWM) is analog, the amplifierarchitecture of the amplifier block 1022 includes pulse width modulationof H-bridge drivers being developed via digital pulse-width modulationby a digital PWM controller 1050. The amplifier architecture of theamplifier block 1022 also includes a closed-loop control feature havinga high loop gain error amplifier and an ADC digitizer.

As described in reference to FIG. 6 , a feedforward digital signal isprovided to the PWM controller 1050 from the digital audio path 1042 ofthe digital logic circuit 1020. More particularly, the feedforwarddigital signal is provided to the PWM controller 1050 from an output ofthe signal limiter block 1146, through a signal path 1043. In FIG. 7 ,such a signal path is also indicated as 1043.

As also described in reference to FIG. 6 , the digital signal from theoutput of the signal limiter block 1146 is also provided to the DAC 1044through the DSM block 1148 and the DEM block 1150. An analog signal fromthe output of the DAC 1044 is utilized as a reference audio signal forthe above-referenced closed-loop of the amplifier block 1022 of FIG. 7 .

Referring to FIG. 7 , it is noted that the feedforward digital audiosignal that is provided to the PWM controller 1050 (through the path1043) is utilized to create most of a signal that determines pulsemodulation for the H-bridge driver 1052. More particularly, the PWMcontroller 1050 is shown to include a pulse generator 1166 thatgenerates control signals HPP_ctrl, HOR_ctrl, HPN_ctrl based mostly onthe feedforward digital audio signal provided through the path 1043 anda mixer 1164. The control signal HPP_ctrl is provided to a ZOR HPPdriver 1170 to generate an analog audio signal HPP when in a ZOR mode;the control signal HOR_ctrl is provided to an HOR driver 1172 togenerate an analog audio signal HPP when in an HOR mode; and the controlsignal HPN_ctrl is provided to a ZOR/HOR HPN driver 1174 to generate ananalog audio signal HPN when in either of the ZOR and HOR modes. Theanalog signals HPP and HPN are shown to drive a speaker 1004 to generatesound.

Referring to FIG. 7 , it is also noted that an error signal generated bythe closed-loop is utilized to develop the remainder of the signal thatdetermines pulse modulation for the H-bridge driver 1052. Such an errorsignal is shown to be provided to the pulse generator 1166 from asuccessive approximation register (SAR) ADC 1048 through a digital loopfilter (DLF) 1162 and the mixer 1164. Such an error signal resultingfrom the closed-loop provides improved audio performance of theamplifier circuit. Examples related to such a closed-loop are describedherein in greater detail.

In the example of FIG. 7 , the H-bridge driver 1052 is shown to includea ZOR HPP driver 1170, an HOR driver 1172 and a ZOR/HOR HPN driver 1174,and a resistance network 1080 (also 1080 in FIG. 6 ) is shown to includea sense resistance (Rs), HOR feedback resistances (Rh1, Rh2) and ZORfeedback resistances (Rz_p, Rz_n). With such drivers, a ZOR mode can beimplemented so that the ZOR HPP (1170) and ZOR/HOR HPN (1174) driversdirectly drive the speaker 1004 (with signals through HPP and HPNnodes), and the ZOR feedback resistances (Rz_p, Rz_n) of the resistancenetwork 1080 directly sense the voltage across the speaker load (HPP andHPN). Accordingly, the ZOR HPP (1170) and ZOR/HOR HPN (1174) drivers aredirectly connected to the HPP and HPN nodes, and thus the speaker load,during the ZOR mode.

Referring to FIG. 7 , an HOR mode can be implemented so that the HORfeedback resistances (Rh1, Rh2) sense a voltage across the senseresistance Rs (e.g., an on-chip current sense resistor), where Rs can beadjusted to be same or close to the resistance (RL) of the speaker load.With such a configuration, the closed-loop operation can force thevoltage signal across the sense resistance Rs to be representative ofthe input signal provided to the H-bridge driver 1052. Accordingly, theresulting current through the speaker load causes an output voltage ofthe H-bridge driver 1052 to be equal to or representative of the inputsignal provided to the H-bridge driver 1052 times the speaker impedance.

It is noted that during the foregoing HOR mode operation, the ZOR HPPdriver 1170 is turned off. It is also noted that the resistance Rs is inseries with the HOR driver 1172 and the HPP node. Accordingly, thespeaker load is driven through the high impedance of the HPP node. TheZOR/HOR HPN driver 1174 drives the other side of the speaker load.

It is noted that the foregoing HOR mode can be utilized to address alow-level electromagnetic-coupled noise problem. For example, the highoutput resistance mode can attenuate the noise at the speaker load. Moreparticularly, in the HOR mode, the speaker load is driven in ahigh-output-impedance mode as a current source mode output instead of avoltage source mode output. Accordingly, the H-bridge driver 1052 forcesa high-fidelity audio current waveform into the speaker load, regardlessof load impedance, nonlinearities and/or noise injections.

The foregoing HOR mode can be calibrated an adjustment of the senseresistance Rs and a digital HOR calibration gain factor applied in oneor more calibration blocks. In some embodiments, such gain calibrationcan be achieved periodically to equal the gain in the ZOR mode. Examplesrelated to such gain calibration are described herein in greater detail.

In the example of FIG. 7 , the PWM amplifier 1022 utilizes modulationfrequency and supply voltage scheme to provide high performance andefficient operation of the H-bridge driver 1052. The modulationfrequency can have a value of several MHz for a pulse width update rateto provide an update period. Such an update period is divided into Nticks utilizing a clock signal. An output pulse width can range from 1to M times the tick width. Accordingly, the output pulse width can havea minimum value of 1×(tick width) and a maximum value of M×(tick width).

Referring to FIG. 7 , the foregoing supply voltage scheme can includeutilization of multiple supply voltages for the output pulses. Forexample, voltages VBAT>VDD_A>VDD_B>VDD_D>VDD_E can be provided to anH-bridge driver supply circuit 1160 for the output voltages. Suchmultiple voltages provided to the output pulses can provide improvedefficiency during operations at different signal levels. For example,for lower level signals, lower voltages can be utilized; and for higherlevel signals, higher voltages can be utilized. To achieve suchfunctionality, the H-bridge driver 1052 can include multiple drivertransistors configured to allow dynamic switching of voltages to any ofthe multiple values based on encoded control signals from the PWMcontroller 1050.

For example, and referring to FIG. 7 , the ZOR HPP and ZOR/HOR HPNdrivers (1170, 1174) are utilized for ZOR mode, and the HOR and ZOR/HORHPN drivers (1172, 1174) are utilized for HOR mode. Depending on themode of operation (ZOR or HOR), amplitude of input signal and PWMencoding rules, the PWM controller 1050 connects the respective driversto one of the available supplies provided through the H-bridge driversupply circuit 1160 to create a zero, positive or negative differentialdrive across the output load nodes HPP and HPN.

As described above, the PWM amplifier 1022 utilizes modulation frequencysuch that a pulse width update is provided during a correspondingperiod. Thus, the PWM controller 1050 can select the supply voltages andpulse width for the respective drivers. For example, the PWM controller1050 can select the largest pulse width and lowest supply possibleduring each update period. Such selections of pulse width and supplyvoltage level can result in the lowest or reduced PWM quantization errorand best or improved power efficiency.

Configured in the foregoing manner, the output of the H-bridge driver1052 can be taken from the differential voltage on the HPP and HPN nodesand directly drive the speaker 1004. It is noted that such an outputdiffers from traditional class D amplifiers in that the foregoing outputappears as high frequency, multi-voltage-level stepping/switchingactivity. Such voltage stepping activity is a notable property of thearchitecture of the PWM amplifier 1022. For the ZOR mode of operationand for audio signals such as sine waves, the voltage level stepping canfollow the envelope of the audio signal. In the HOR mode of operation,the HPP and HPN single-ended output switching appears different than inthe ZOR mode. The behavior of the signal on the HPP and HPN nodes candepend on the polarity of the output signal. When measuring the HPP andHPN nodes single-ended to ground, during parts of the output signalcycle, the HPP and HPN waveforms may not resemble the audio envelope inthe same way as in the ZOR mode. Such a difference can result from theHOR mode's selection of HPP node voltage that forces the PWM controller1050 to produce switching behavior that holds the HPP node high.Therefore, the HPP node is held high for a significant part of thewaveform cycle while the HPN node is switching.

In the PWM amplifier 1022 of FIG. 7 , the closed-loop can be configuredand operated as follows. As described herein, such a closed-loop canprovide an error signal that is utilized for development of a signalthat determines pulse modulation for the H-bridge driver 1052, therebyproviding improved audio performance of the amplifier circuit.

Referring to FIG. 7 , the outputs HPP and HPN of the H-bridge driver1052 can be fed back to the summing nodes 1081, 1083 through theresistance network 1080. More particularly, the output node HPP is shownto be coupled to the summing node 1081 through a resistance Rz_p and arespective mixer also being provided with a ZOR mode signal as an input.Thus, the output of the mixer is shown to be added with a respectivereference output of the DAC 1044 at the summing node 1081; and thesummed signal is shown to be provided to the loop filter 1046.Similarly, the output node HPN is shown to be coupled to the summingnode 1083 through a resistance Rz_n and a respective mixer also beingprovided with a ZOR mode signal as an input, such that the output of themixer is shown to be added with a respective reference output of the DAC1044 at the summing node 1083; and the summed signal is shown to beprovided to the loop filter 1046.

Configured in the foregoing manner, the loop filter 1046 is providedwith a signal representative of a differential error between the outputs(HPP, HPN) of the H-bridge driver 1052 and the reference signal providedby the DAC 1044. In the example of FIG. 7 , the each of the summingnodes 1081, 1083 is shown to be provided with a signal from thecommon-mode limiting (CML) amplifier 1082. Such signals from the CMLamplifier 1082 can be utilized to limit the input common-mode voltage ofthe loop filter 1046.

In the example of FIG. 7 , the loop filter 1046 can include a 5th orderhigh gain loop filter to provide an output to a low-power, high-speedSAR ADC 1048 to digitize the loop filter output. The digitized output ofthe SAR ADC 1048 is shown to be provided to the PWM controller 1050,where it is utilized by the PWM controller 1050 along with thefeedforward signal (provided through the path 1043) to generate PWMcontrol signals for the H-bridge driver 1052.

Referring to FIG. 7 , it is noted that in the foregoing closed-loop,without any compensation, an inductance of the speaker's driver elementcan lead to significant differences in the open-loop frequency responseat high frequencies (e.g., greater than 100 KHz) between the HOR and ZORmodes. Such an effect is due to the current-mode drive of the senseresistance working into the frequency dependent impedance of the speakerinductance during the HOR mode of operation.

In the closed-loop circuit of FIG. 7 , the loop includes the loop filter1046, SAR ADC 1048, PWM controller 1050, H-bridge driver 1052 andresistance network 1080. In addition to such components, a digital loopfilter (DLF) 1162 can be provided to provide compensation for loopstability in situations where processing delays are present. In someembodiments, such a DLF can be configured to provide compensation byinsertion of a programmable digital filter that includes parallelarrangement of finite impulse response (FIR) and infinite impulseresponse (IIR) sections, between the SAR ADC 1048 and the normal inputlocation (mixer 1164 in FIG. 7 ). Such a DLF can be configured toprovide a response that includes phase compensation in response to theeffect of the inductor in the HOR mode, as well as shaping of one ormore characteristics of the closed-loop.

As described herein, when the PWM amplifier 1022 of FIG. 7 is in ZORmode, the gain is determined by the resistances of the resistancenetwork 1080 working against the DAC (1044) output current. The feedbackresistances (Rz_p, Rz_n) sense the voltage across the HPP and HPN nodes,and provides feedback to the loop filter inputs. The closed-loop withsuch a feedback can force the output of the PWM amplifier 1022 to beadjusted to equal or approximately equal the digital input with a netgain (e.g., G=1 such that a 0 dBFS input produces a 0 dBv output).

In HOR mode, however, the gain is determined differently, since the HORmode utilizes a current-mode output where the signal current is producedacross the sense resistance (Rs) and forced through the load resistance(Rload) of the speaker (1004 in FIG. 7 ). The HOR feedback resistances(Rh1, Rh2) sense the voltage across the sense resistance Rs, and such asensed voltage works against the DAC output current. The closed-loopwith such a feedback can force the output current gain of the PWMamplifier 1022 to be adjusted to be G=1/Rs. If there is no furtheradjustment, then the net end-to-end gain in the HOR mode would beG=Rload/Rs. If Rs and Rload are not equal to each other, then the HORgain will not be equal to the gain in ZOR mode (G=1). To address such aneffect, an audio amplifier circuit as described herein can configured toinclude an HOR gain calibration functionality utilizing, for example,adjustment of the sense resistance and a digital gain term to make thegain in HOR mode equal to or approximately equal to the gain in the ZORmode. Examples related to such a gain calibration functionality aredescribed herein in greater detail.

FIG. 8 shows a block diagram of a digital logic circuit block 1020 thatis a more specific example of the digital logic circuit block 1022 ofFIG. 5 , implemented to operate with the H-bridge driver 1052 of FIG. 7to provide functionalities including mode switching between HOR and ZORmodes. Such mode switching can be achieved as follows.

It is noted that the audio amplifier circuit as described herein canprovide dynamic switching between HOR and ZOR modes. Such mode-switchingoperation can include switching of the feedback resistanceconfigurations of the resistance network (1080 in FIG. 8 ) between theoutput nodes (HPP, HPN) to the loop filter block (1046). Suchmode-switching operation can be achieved in a dynamic manner with lowaudio artifacts. However, when switching between modes, because of thecomplex impedance of the speaker driver element and the current-modeoperation in the HOR mode, the end-to-end frequency response of thesystem may change. If such a difference in frequency response is notcompensated, the change may result in audible artifacts. To compensatefor the change in frequency response, the audio amplifier circuit asdescribed herein can include a digital EQ filter to allow compensationof the frequency response difference between the two modes.

In addition, to minimize the audible artifacts during HOR/ZORtransitions, the audio amplifier circuit as described herein can includea number of features. For example, the resistance network 1080 can becontrolled to provide stepped output resistance Rout. In anotherexample, a HOR/ZOR EQ block 1142 can be configured to operate with thestepped Rout values. In yet another example, a HOR/ZOR state controlblock 1062 can be provided and configured to control the HOR/ZORtransitions.

It is noted that an abrupt transition in the output resistance Rout seenby the speaker driver during HOR/ZOR transitions can cause asufficiently large phase shift to be audible. To reduce or eliminatesuch audible artifacts, an amplifier equivalent Rout can be made totransition more gradually by moving through a series of Rout steps(e.g., six Rout steps) during a transition between HOR and ZOR modes. Aset of particular Rout step values can be selected by selected values offeedback resistances Rh and Rz of the resistance network 1080. Such Routstep values can be selected to produce approximately equal phaseartifacts error per step. Further, time duration per step can beprogrammable over a modest range. Given a non-linear relationshipbetween phase error and step size, Rout stepping as described herein canprovide a significant impact on the reduction in the audibility of theartifacts.

As described in reference to FIG. 6 , an HOR/ZOR EQ block 1142 can beprovided as part of the digital audio path 1042. FIG. 8 shows that suchan HOR/ZOR EQ block (also 1142) can be implemented to operate with theforegoing Rout stepping functionality.

Referring to FIG. 8 , the HOR/ZOR EQ block 1142 is shown to be driven bythe output of the PDM LPF block 1140. The HOR/ZOR EQ block 1142 can beconfigured to provide filtering to compensate for the difference betweenthe HOR and ZOR mode frequency responses driving the speaker transducer.The HOR/ZOR EQ block 1142 is shown to include an EQ filter bank, gaincompute block and cross-fading functionality.

More particularly, the EQ filter bank is shown to include three parallelfilter sections 1200 (Spare BQ), 1202 (Shelving) and 1204 (Bandpass)configured to compensate the frequency response in the speakertransducer. The filter section 1200 (Spare BQ) can be implemented as alow frequency 2nd order (biquad) bandpass filter section (e.g., up to 1KHz). The filter section 1202 (Shelving) can be implemented as a 4thorder finite impulse response (FIR) shelving filter. The filter 1204(Bandpass) can be implemented as a 2nd order general purpose biquadfilter.

Referring to FIG. 8 , the EQ block 1142 can be configured to optionallyprovide filtering to compensate for the difference between the HOR andZOR mode frequency responses driving the speaker transducer. In additionto the foregoing EQ filter bank and the cross-fading functionality, theEQ block 1142 can also include a gain compute block 1212. FIG. 9 shows amore specific configuration of such a gain compute block.

Referring to FIGS. 8 and 9 , the EQ block 1142 is shown to providetapered in and out via a crossfading mixing structure of gains G1 and G2(through mixers 1208, 1210) and an output adder 1214.

More particularly, gain G1 is shown to be associated with mixing of anunfiltered output of the PDM LPF block 1140 with a non-EQ gain signalfrom the gain compute block 1212 at the mixer 1208, and gain G2 is shownto be associated with mixing of a summed filtered signal with an EQ gainsignal from the gain compute block 1212 at the mixer 1210. The foregoingsummed filtered signal is shown to be obtained by outputs of the filtersections 1200, 1202, 1204 being added by an adder 1206. The input toeach of the filter sections 1200 (Spare BQ) and the filter section 1202(Shelving) is shown to be provided from the output of the PDM LPF block1140, and the input to the filter section 1204 (Bandpass) is shown to beprovided from the input of the PDM LPF block 1140.

Referring to FIGS. 8 and 9 , the gain value G1 is associated with anon-filtered signal, and the gain value G2 is associated with a filteredsignal. The gains G1 and G2 can be values ranging from 0 to 1, whereG1+G2=1, and in the example context of Rout having six steps, each of G1and G2 can have six discrete step values of 0, 0.2, 0.4, 0.6, 0.8 and 1.As described herein, such gain values can be stepped with the Routstepping of the audio amplifier circuit.

Referring to FIG. 9 , the gain compute block 1212 is shown to beconfigured to generate the non-EQ gain signal (to the mixer 1208 in FIG.8 ) and the EQ gain signal (to the mixer 1210) based on a number ofinputs. For example, the gain compute block 1212 can handle slowlychanging gain parameters such as HOR/ZOR gain, user gain, HORcalibration gain, and EQ biquad gain. Based on some or all of suchinputs, and depending on the operating mode (ZOR or HOR), the gaincompute block 1212 can compute net EQ path gain and non-EQ path gainvalues and provide such gain values as outputs.

More particularly, the user gain is shown to be processed through a gainramp 1220 and provided to a mode gain multiplexer 1222 that alsoreceives a HOR mode signal. The mode gain multiplexer 1222 is shown togenerate ZOR gain and HOR gain and provide such outputs to a multiplexer1226, with the latter being mixed with a HOR calibration gain value by amixer 1224. The multiplexer 1226 is shown to provide the non-EQ gain andEQ gain values based on the ZOR gain and HOR gain values, along with anEQ select input. The EQ gain value is shown to result from mixing of therespective output of the multiplexer 1226 with the EQ biquad gain by amixer 1228.

Configured in the foregoing manner, the gain compute block 1212 canprovide some or all of the following functionalities: provide a gainstage for digital audio gain adjustment, provide an amplifier stateinitiated gain ramp-up after start of audio, provide an amplifierinitiated gain ramp-down before shutdown of audio, provide a gain stagefor HOR/ZOR fine gain calibration (e.g., HOR calibration gain) where thefine gain calibration can be applied in all HOR modes (full andpartial), provide gain control for a programmable ramp time, and providea test mode programmable volume gain register to mute gain block with avariable attenuation step size to provide a desired dynamic range.

Configured as described above in reference to FIGS. 8 and 9 , theHOR/ZOR EQ block 1142 can provide some or all of the followingfunctionalities: enabling or bypassing of EQ functionality, compensatingfor transducer impedance vs frequency with a multi-section filter,switching in and out of the audio path with minimal or reduced audioartifacts, tracking of output resistance Rout, programmable filtercoefficients, minimized or reduced latency, and provide EQ filteringactive for HOR or ZOR mode and off for the other.

FIG. 8 also shows an HOR/ZOR state control block 1062 that can be a morespecific example of the HOR/ZOR state control block 1062 of FIGS. 5 and6 . In the example of FIG. 8 , the HOR/ZOR state control block 1062 candetermine when to switch between HOR and ZOR modes by observing theaudio signal path. Such a switching can be controlled so that audibleartifacts due to switching are minimized or reduced. In someembodiments, HOR/ZOR switching can be configured to maximize or increasetime in the ZOR mode for efficiency, but prioritize HOR mode when audiosignal is low in order to minimize or reduce injected interference. TheHOR/ZOR state control block 1062 can include an HOR/ZOR Rout modecontrol block 1190 that utilizes, for example, psychoacousticprinciples, to mask or reduce transient artifacts during mode switchingoperations.

Referring to FIG. 8 , the HOR/ZOR state control block 1062 can providesome or all of the following functionalities. First, the audio amplifiercircuit can be placed in HOR mode when amplitude of the digital audiosignal (e.g., obtained from the input side of the PDM LPF 1140) is lowin order to minimize or reduce injected interference. Second, the audioamplifier circuit can be placed in ZOR mode when amplitude of thedigital audio signal is high in order to improve efficiency. Third,transition time from ZOR to HOR mode can be made to be within a selectedtime duration. Fourth, Rout transitions can be made without or reducedaudible artifacts. Fifth, HOR/ZOR target mode can be determined by anaverage audio level. Sixth, HOR/ZOR switching time can be set to be whenthe transition will be inaudible as determined by a combination ofconfigurable factors such as crest factor, audio signal level beingbelow a threshold value, and time from the last Rout change. Seventh,HOR/ZOR output impedance can have multiple (e.g., 6) steps correspondingto the Rout modes (e.g., Rour0, Rout1, Rout2, Rout3, Rout4, Rout5).Eighth, HOR/ZOR impedance steps can be controlled by moving from currentimpedance step toward the target impedance step when transitions aredetermined to be inaudible. Ninth, HOR/ZOR transitions can beprogrammable to step through discrete output impedances in order tominimize or reduce switching transients (e.g., output impedance stepcontrols being implemented to correspond to output impedance steps ofthe audio amplifier). Tenth, HOR/ZOR transitions can be independentlyconfigured to occur with a minimum or reduced time between switching.Eleventh, HOR/ZOR mode can have a manual override configured by resisteraccess, and such an override can be configured to allow setting of allmode and/or Rout settings.

In the example of FIG. 8 , switching between HOR and ZOR modes can beachieved as follows. Switching between HOR and ZOR modes can includeoperations of the resistance network 1080, the HOR/ZOR EQ block 1142,and the HOR/ZOR state control block 1062.

Referring to FIG. 8 and the foregoing HOR/ZOR switching functionality,the resistance network 108 can be utilized to provide an Rout steppingfunctionality as described herein. It is noted that an abrupt transitionin Rout seen by the speaker driver during HOR/ZOR transitions can causea sufficiently large phase shift to be audible. To reduce such audibleartifacts, an amplifier equivalent Rout can be made to transition moregradually be moving through a number of Rout steps (e.g., 6 steps)during a transition between HOR and ZOR modes. Such stepped Rout valuescan be achieved through selection of resistance values for Rh and Rzimplemented as, for example, variable resistors. The equivalent Routsteps can be selected to produce approximately equal phase artifacterror per step, and the time per step can be programmed over a selectedrange. Given the non-linear relationship between phase error and stepsize, Rout stepping functionality can provide a significant impact onthe reduction in the audibility of the artifacts during HOR/ZORtransitions.

Referring to FIG. 8 and the foregoing HOR/ZOR switching functionality,the HOR/ZOR EQ block 1142 can be configured to provide EQ filteringfunctionality including EQ filtering functionality for compensation ofthe difference in frequency responses between HOR and ZOR modes asdescribed herein. The HOR/ZOR EQ block 1142 can be configured to operatewith such EQ filtering functionality during the HOR or ZOR mode. In someembodiments, some or all of the EQ block 1142 can be disabled in the HORmode to save power, and be enable during the ZOR mode if desired orneeded. It is noted that if the foregoing Rout stepping functionality isutilized, the EQ filter response can be synchronized to be graduallystepped with a corresponding change in the Rout.

Referring to FIG. 8 and the foregoing HOR/ZOR switching functionality,the HOR/ZOR state control block 1062 can be configured to decide when toswitch between HOR and ZOR modes by monitoring the audio signal path.For example, audio signal path before and after the PDM LPF block 1140can be monitored.

Referring to FIG. 8 , the node on the input side of the PDM LPF block1140 is shown to be coupled to a decimation circuit 1180 configureddecimate the sampled signal by N (e.g., N=128). The decimated signal isthen shown to be provided to a block 1182 for low audio detection, ablock 1184 for generating a target mode, and a block 1186 for selectingan Rout step. Outputs of the blocks 1182, 1184, 1186 are shown to beprovided to a HOR/ZOR Rout mode control block 1190. Referring to FIG. 8, the node on the output side of the PDM LPF block 1140 is shown to becoupled to an audio switch block 1188 that provides its output to theHOR/ZOR Rout mode control block 1190.

Configured in the foregoing manner, the HOR/ZOR state control block 1062can determine when to switch between HOR and ZOR modes. Suchdetermination can be based on some or all of a number of conditions. Forexample, an average of the sampled input signal can be obtained andcompared to a threshold value. More particularly, a determination ofwhether the average of the sampled signal is greater than a HOR-to-ZORthreshold value can be made when in HOR mode, or whether the average ofthe sampled signal is less than a ZOR-to-HOR threshold value can be madewhen in ZOR mode. In another example, a determination of whether theinput signal has a very low level can be made. In yet another example,the input signal can be passed through a high-pass filter, and a peak insuch a filtered signal can be compared to a product of an average of thefiltered signal and a crest factor. If the peak value is greater thanthe product, and if the input signal is at or near a zero crossing, adetermination can be made to perform a mode-switching operation.

It is noted that the foregoing HOR-to-ZOR threshold value and ZOR-to-HORthreshold value may or may not be different.

For a HOR-to-ZOR transition, determination can be made as to whether theinput signal level has crossed a respective threshold level, and whethera favorable transition condition (e.g., high crest factor, higherfrequency masking event and a subsequent zero crossing). If so, theHOR-to-ZOR transition can be made to proceed. For a ZOR-to-HORtransition, determination can be made as to whether the input signallevel has crossed a respective threshold level or has becomesufficiently small, and whether a favorable transition condition (e.g.,high crest factor, higher frequency masking event and a subsequent zerocrossing). If so, the ZOR-to-HOR transition can be made to proceed. Theforegoing transition techniques allow the audio amplifier to operate todeliver full output power in either HOR or ZOR modes, thus enabling theaudio amplifier to remain in either mode until a favorable transitioncondition is present to thereby avoid audible artifacts.

As described herein, HOR mode gain is determined differently than ZORmode gain. However, it is desirable to operate an audio amplifiercircuit as described herein so that a net gain in the HOR mode isequivalent to a net gain in the ZOR mode. In some embodiments, HOR gaincan be adjusted so to be equivalent to ZOR gain. In various examplesdescribed herein, such a ZOR gain can be considered to have a gain G=1;thus, HOR gain can be calibrated to also provide a gain G=1. Such a HORgain calibration can be achieved as follows.

FIG. 10 shows various functional blocks of the audio amplifier circuit1000 of FIG. 5 , where such blocks can form an HOR gain calibrationsub-system. More particularly, such a sub-system can include a gaincalibration block 1070, a gain calibration ADC 1084, an Rout controller1064, a resistance network 1080, and an HOR/ZOR EQ block 1142. FIG. 11shows a more detailed example of the gain calibration block 1070 of FIG.10 .

Referring to FIGS. 10 and 11 , the gain calibration block 1070 can beimplemented as a digital block that includes a gain calibrationprocessor 1230 and a gain calibration controller 1238. The gaincalibration processor 1230 can be configured to perform gain estimationcomputation for HOR mode, and based on such gain estimation, desiredsense resistance (Rs in FIGS. 7 and 8 ) value and digital gaincorrection value can be determined. The gain calibration controller 1238can handle overall management of the gain calibration sub-system,including determining when to calibrate and/or determining conditionsfor a valid calibration.

Referring to FIGS. 10 and 11 , the gain calibration ADC 1084 can beimplemented as an analog block that digitizes the differential amplifieroutput HPP-HPN during a calibration cycle to use by the foregoing gaincalibration processor 1230. The gain calibration ADC 1084 can beconfigured (e.g., with delta-sigma modulation) to handle large audiosignals at the output (HPP, HPN) and have a sufficiently large dynamicrange to detect the relatively lower amplitude calibration tone signal.

Referring to FIG. 11 , the gain calibration processor 1230 is shown toinclude a calibration tone generator 1232 configured to generate anultrasonic calibration tone (Cal. tone) which is mixed into the digitalaudio stream with the mixer 1144 before the signal limiter block 1146.The tone generator 1232 can also be configured to provide programmablefrequency and amplitude functionalities. In some embodiments, the tonegenerator 1232 can produce a sinusoidal output modulated with, forexample, a 2nd order cascaded integrator-comb (CIC) filtered pulse toprevent audible energy during turning on and off of the tone generator1232.

Referring to FIGS. 10 and 11 , the Rout controller 1064 can beimplemented as a digital block that performs computations to determineand control the sense resistance (Rs) setting in the analog resistancenetwork 1080. The Rout controller 1064 can also perform computations todetermine a digital fine gain value that is provided to the HOR/ZOR EQblock 1142 for use in a fine gain adjustment.

Referring to FIGS. 10 and 8 , the resistance network 1080 can beimplemented as an analog block that includes a programmable senseresistance (Rs) network, and programmable ZOR and HOR Rout mode feedbackresistances (Rz and Rh). Such a resistance network is shown to becontrolled by the Rout controller 1064.

Referring to FIGS. 10 and 11 , and as described above, the HOR/ZOR EQblock 1142 and its gain compute block (1212 in FIGS. 8 and 9 ) canprovide a gain multiplier functionality where HOR digital calibrationgain is applied. Such an application of the HOR digital calibration gaincan be based on a control signal (HOR Cal fine gain) provided by theRout controller 1064.

Configured in the foregoing manner, the gain calibration processor 1230can compute a gain mismatch between HOR and ZOR modes by introducing acalibration tone (e.g., an ultrasonic tone at ˜25 KHz) into the digitalaudio stream at the mixer 1144 before the signal limiter block 1146. Thedigital audio stream with the calibration tone mixed therein is passedthrough the signal limiter block 1146. At the output of the signallimiter block 1146, the digital audio stream is routed through path 1043to the PWM controller (1050 in FIG. 8 ) to be processed and amplified bythe H-bridge driver 1052 to provide an output at HPP and HPN. Also atthe output of the signal limiter block 1146, the digital audio stream isobtained for the gain calibration processor 1230 as a reference signal.Thus, the magnitude and phase of the output signal at the output (HPP,HPN) can be compared relative to the magnitude and phase of thereference signal.

To achieve the foregoing comparison of the output signal (analog signal)with the reference signal (digital signal), the gain calibration ADC1084 can sample the output voltage across the load (HPP-HPN) and providea delta-sigma ADC output to the gain calibration processor 1230.Measurement and computation of the HOR and ZOR gains can be performed bythe gain calibration processor 1230 (e.g., with a gain compute block1236) utilizing an estimation algorithm where the reference signal X andthe digitized output signal Y are downconverted with a tone at the samefrequency as the calibration tone (e.g., 25 KHz). The downconvertedsignals X and Y can be filtered (e.g., single-bin fast Fourier transform(FFT) with a discrete Fourier transform (DFT) block 1234) to providerespective complex downconverted values x and y. A ratio of the twocomplex downconverted values can be obtained, where Ratio=y/x. It isnoted that complex values can be utilized so that the load inductancedoes not significantly affect the gain calculation; however, the realcomponent of the Ratio is utilized.

The foregoing Ratio=y/x is an expression of a transfer function gainfrom the digital input to the amplifier output, and can be designed tobe tolerant to out-of-band interference. Ratio can be computed for eachof the HOR and ZOR modes, such that Ratio(HOR)=y/x in the HOR mode, andRatio(ZOR)=y/x in the ZOR mode.

The calibration processor 1230 can then compute another ratioRelative_HOR_gain=Ratio(HOR)/Ratio(ZOR) which is representative of theHOR gain relative to the ZOR gain. Ideally, this ratio Relative_HOR_gainhas a value of 1.

Referring to FIG. 11 , the computed Relative_HOR_gain value is shown tobe provided to the Rout control block 1064 by the gain calibrationcontroller 1238. Based on such a computed Relative_HOR_gain value, theRout control block 1064 can determine an adjustment to the senseresistance (Rs) and a fine gain (HOR Cal fine gain in FIG. 10 ) controlsignal to be applied to the digital path by the gain compute block 1212of the HOR/ZOR EQ block 1142.

More particularly, the Rout control block 1064 can obtain theRelative_HOR_gain value and perform computations to determine how tochange the sense resistance (Rs) relative to its present setting. Basedon the change it makes to the sense resistance (Rs) setting, the Routcontrol block 1064 can compute the digital fine gain adjustment neededto make the Relative_HOR_gain value to be 1.

A PWM amplifier as described herein can be configured to operate withmultiple supply voltages to provide desired performance for both low andhigh level signals. For example, the H-bridge driver 1052 of the PWMamplifier 1022 in each of FIGS. 5 and 7 is shown to be provided withsupply voltages VBAT, VDD_A, VDD_B, VDD_D and VDD_E, withVBAT>VDD_A>VDD_B>VDD_D>VDD_E. In such an example PWM amplifier, the fourlarger voltages (VBAT, VDD_A, VDD_B, VDD_D) can be obtained from one ormore external sources, and the smallest voltage (VDD_E) can be obtainedfrom an internal source.

In some embodiments, the foregoing internal source voltage (VDD_E) canbe obtained from a low voltage monitor (LVM) block indicated as 1132 ineach of FIGS. 5 and 7 . In some embodiments, such a voltage (VDD_E) canbe provided to the H-bridge driver 1052. In some embodiments, thevoltage VDD_E can also be provided to an external location through avoltage node indicated as VDD_E.

FIG. 12 shows a circuit diagram of a low voltage system 100 that can beutilized in an audio amplifier having one or more features as describedherein. In some embodiments, such a system can include a low voltagemonitoring circuit 102 (1132 in FIG. 7 ), a controller 110 (1050 in FIG.7 ), an H-bridge driver circuit 112 (1052 in FIG. 7 ), and a capacitorC. As described herein, such a system can allow low voltage operation ofan audio amplifier, such as the PWM amplifier 1022 of FIG. 7 , without aneed for an external low voltage supply. It will be understood thatwhile various examples are described herein in the context of such a PWMamplifier, one or more features of the present disclosure can also beimplemented with other types of amplifiers, including other types ofaudio amplifiers.

In FIG. 12 , the H-bridge driver 112 is shown to be provided with asupply voltage VDD1. As described herein, such a voltage (VDD1) can bethe lowest supply voltage (VDD_E in FIG. 7 ). In some embodiments, sucha supply voltage (VDD1) can be maintained with use of the capacitor Cand charging/discharging of the capacitor C based on monitoring of thesupply voltage VDD1 as described herein.

FIG. 12 shows that in some embodiments, the low voltage monitoringcircuit 102 can include a comparator 106 that compares a voltage at anoutput node 120 of a P/N driver 104 with a reference voltage Vref. Theoutput node 120 is also shown to be connected to the supply voltage nodeVDD1.

In FIG. 12 , the P/N driver 104 is shown to include an arrangement of aPMOS device having its source coupled to a supply voltage (VDD2) nodeand its drain coupled to the drain of an NMOS device. The source of theNMOS device is shown to be coupled to a ground. The gate of the PMOSdevice is shown to be provided with a control signal UP, and the gate ofthe NMOS device is shown to be provided with a control signal DN. Thus,the P/N driver 104 can provide an output voltage at the node 120 basedon the control signals UP and DN.

In the example of FIG. 12 , the supply voltage VDD2 for the P/N driver104 is greater than the supply voltage VDD1.

Referring to FIG. 12 , an output of the comparator 106 is shown to beprovided to the PWM controller 110. Such a controller is shown toprovide the UP and DN control signals to the P/N driver 104 based on theoutput of the comparator 106.

In FIG. 12 , the PWM controller 110 is also shown to provide controlsignal Ctrl_R for controlling a resistance R of a variable resistor of areference voltage circuit. Such a reference voltage circuit is shown toinclude a current source 108 coupled to ground through the resistor R,with the reference voltage Vref being obtained from a node between thecurrent source 108 and the resistor R.

In FIG. 12 , the PWM controller 110 is also shown to provide a controlsignal (PWM code) to the H-bridge driver 112. The H-bridge driver 112 isshown to be coupled to the supply node VDD1 (which is also the outputnode 120 of the P/N driver 104) so as to allow the speaker 114 to bedriven according to the control signal (PWM code).

In FIG. 12 , the capacitor C is shown to couple the output node 120 ofthe P/N driver 104 to ground. In some embodiments, the capacitor C canbe an external capacitor that is outside of a die having the PWMamplifier. In some embodiments, the capacitor C can be implemented aspart of a die having the PWM amplifier.

Configured in the foregoing manner, the low voltage system 100 of FIG.12 can provide a low voltage supply for use in low level operation ofthe H-bridge driver 112. Such a functionality reduces the powerconsumption associated with regulation of a low voltage, since the powerthat would normally be lost in regulation is output to the load instead.It is noted that PWM Class D operation on a low supply reduces switchinglosses proportional to CV², and also reduces noise in the system.

In the system 100 of FIG. 12 , low voltage regulation can be achieved bymonitoring the voltage on the capacitor C (at the output node 120) andconfiguring switching operations of the H-bridge driver 112 to charge ordischarge the capacitor C, depending on whether the monitored voltage ishigher or lower than a selected value representative of a desired valuefor the supply voltage VDD1. Such a selected value can be programmableand/or dynamic if desired. In some embodiments, a programmable selectedvalue can allow control of an idle output common mode of the amplifier.

As described herein, the low voltage monitoring circuit 102 monitors thevoltage of the capacitance C at the output node 120 to determine if itis higher or lower than the reference voltage Vref. In some embodiments,such a reference voltage can be a voltage from a software controllablereference source.

In an example operation, during each PWM cycle, the programmablereference voltage can be stepped to track the voltage on the capacitor Cin real or approximately real time. The reference voltage Vref can berealized with a fixed current into the variable resistance R that, insome embodiments, can be implemented as a programmable resistor ladder;and such a fixed current through the resistance R can generate thereference voltage Vref being provided to the comparator 106.

If the voltage on the capacitor C is high, next output pulse can begenerated by connecting the load between the capacitor C and ground,thereby reducing the voltage on the capacitor C. If the voltage on thecapacitor is low, the next output pulse can be generated by connectingthe load between the capacitor C and the lowest available supply (VDD1),thereby increasing the voltage on the capacitor C.

In some embodiments, an extension on the foregoing operation can add aprogrammable amount to any “charge pulse” to deliver more charge to thecapacitor C per pulse, which improves the ratio of charge delivered vspower dissipated switching the output stage.

In the example of FIG. 12 , the foregoing charging and discharging ofthe capacitor C can be achieved by application of the respective pulses(e.g., PWM codes) to the H-bridge driver 112. Accordingly, the chargingand discharging of the capacitor C to maintain a desire value of thesupply voltage VDD1 are collectively indicated as 122.

FIG. 13 shows the low voltage system 100 of FIG. 12 being operated withspecific example settings. In FIG. 13 , the capacitor C has acapacitance value of approximately 300 nF, the PMOS device has aneffective resistance of approximately 200 ohm, and the NMOS device hasan effective resistance of approximately 25 ohms.

FIG. 14 shows examples of various signal traces during the operation ofthe low voltage system 100 of FIG. 13 .

Referring to FIGS. 13 and 14 , examples of switching operation caninvolve some or all of the following situations. If a positive drive isneeded and the capacitor C needs to be charged, the amplifier's positiveoutput (out+) can be connected to the lowest available external supply,and the negative output (out−) can be connected to the capacitor C. InFIG. 14 , such an operating configuration is depicted on the left sideof the charging case, where the example lowest available external supplyis VDD_D, with VDD_D>VDD_E.

If a negative drive is needed and the capacitor C needs to be charged,out− can be connected to the lowest available external supply, and out+can be connected to the capacitor C. In FIG. 14 , such an operatingconfiguration is depicted on the right side of the charging case.

If a positive drive is needed and the capacitor C needs to bedischarged, out+ can be connected to the capacitor C, and out− can beconnected to ground. In FIG. 14 , such an operating configuration isdepicted on the left side of the discharging case.

If a negative drive is needed and the capacitor needs to be discharged,out− can be connected to the capacitor C, and out+ can be connected toground. In FIG. 14 , such an operating configuration is depicted on theright side of the discharging case.

In some situations, the system may require additional decision makingcriteria, not all switching options may be available at all times, andthe low voltage operation cannot be guaranteed to stay within regulationlimits over all conditions. To ensure that the supply voltage stayswithin a desired range, a pullup or pulldown switch can be activated toreturn the system to regulation.

In some embodiments, if the pullup or pulldown is activated for too manycycles in a row, the system can be assumed to have lost regulation. Insuch a situation, the low voltage operation can be automaticallydisabled, and the lowest externally available supply can be utilized.

In some situations, there may be output conditions that move the voltageon the capacitor C faster than the reference can track. In someembodiments, a modified mode of the low voltage system can allow thereference to be moved by more than one code per cycle in order to trackthe voltage when high current is in the load. If the code needs tochange in the same direction for a programmable number of consecutivecycles, the modified mode can be enabled and the code can be changed bya programmable amount each cycle until the reference catches up.

The present disclosure describes various features, no single one ofwhich is solely responsible for the benefits described herein. It willbe understood that various features described herein may be combined,modified, or omitted, as would be apparent to one of ordinary skill.Other combinations and sub-combinations than those specificallydescribed herein will be apparent to one of ordinary skill, and areintended to form a part of this disclosure. Various methods aredescribed herein in connection with various flowchart steps and/orphases. It will be understood that in many cases, certain steps and/orphases may be combined together such that multiple steps and/or phasesshown in the flowcharts can be performed as a single step and/or phase.Also, certain steps and/or phases can be broken into additionalsub-components to be performed separately. In some instances, the orderof the steps and/or phases can be rearranged and certain steps and/orphases may be omitted entirely. Also, the methods described herein areto be understood to be open-ended, such that additional steps and/orphases to those shown and described herein can also be performed.

Some aspects of the systems and methods described herein canadvantageously be implemented using, for example, computer software,hardware, firmware, or any combination of computer software, hardware,and firmware. Computer software can comprise computer executable codestored in a computer readable medium (e.g., non-transitory computerreadable medium) that, when executed, performs the functions describedherein. In some embodiments, computer-executable code is executed by oneor more general purpose computer processors. A skilled artisan willappreciate, in light of this disclosure, that any feature or functionthat can be implemented using software to be executed on a generalpurpose computer can also be implemented using a different combinationof hardware, software, or firmware. For example, such a module can beimplemented completely in hardware using a combination of integratedcircuits. Alternatively or additionally, such a feature or function canbe implemented completely or partially using specialized computersdesigned to perform the particular functions described herein ratherthan by general purpose computers.

Multiple distributed computing devices can be substituted for any onecomputing device described herein. In such distributed embodiments, thefunctions of the one computing device are distributed (e.g., over anetwork) such that some functions are performed on each of thedistributed computing devices.

Some embodiments may be described with reference to equations,algorithms, and/or flowchart illustrations. These methods may beimplemented using computer program instructions executable on one ormore computers. These methods may also be implemented as computerprogram products either separately, or as a component of an apparatus orsystem. In this regard, each equation, algorithm, block, or step of aflowchart, and combinations thereof, may be implemented by hardware,firmware, and/or software including one or more computer programinstructions embodied in computer-readable program code logic. As willbe appreciated, any such computer program instructions may be loadedonto one or more computers, including without limitation a generalpurpose computer or special purpose computer, or other programmableprocessing apparatus to produce a machine, such that the computerprogram instructions which execute on the computer(s) or otherprogrammable processing device(s) implement the functions specified inthe equations, algorithms, and/or flowcharts. It will also be understoodthat each equation, algorithm, and/or block in flowchart illustrations,and combinations thereof, may be implemented by special purposehardware-based computer systems which perform the specified functions orsteps, or combinations of special purpose hardware and computer-readableprogram code logic means.

Furthermore, computer program instructions, such as embodied incomputer-readable program code logic, may also be stored in a computerreadable memory (e.g., a non-transitory computer readable medium) thatcan direct one or more computers or other programmable processingdevices to function in a particular manner, such that the instructionsstored in the computer-readable memory implement the function(s)specified in the block(s) of the flowchart(s). The computer programinstructions may also be loaded onto one or more computers or otherprogrammable computing devices to cause a series of operational steps tobe performed on the one or more computers or other programmablecomputing devices to produce a computer-implemented process such thatthe instructions which execute on the computer or other programmableprocessing apparatus provide steps for implementing the functionsspecified in the equation(s), algorithm(s), and/or block(s) of theflowchart(s).

Some or all of the methods and tasks described herein may be performedand fully automated by a computer system. The computer system may, insome cases, include multiple distinct computers or computing devices(e.g., physical servers, workstations, storage arrays, etc.) thatcommunicate and interoperate over a network to perform the describedfunctions. Each such computing device typically includes a processor (ormultiple processors) that executes program instructions or modulesstored in a memory or other non-transitory computer-readable storagemedium or device. The various functions disclosed herein may be embodiedin such program instructions, although some or all of the disclosedfunctions may alternatively be implemented in application-specificcircuitry (e.g., ASICs or FPGAs) of the computer system. Where thecomputer system includes multiple computing devices, these devices may,but need not, be co-located. The results of the disclosed methods andtasks may be persistently stored by transforming physical storagedevices, such as solid state memory chips and/or magnetic disks, into adifferent state.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Additionally, the words “herein,” “above,” “below,” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Wherethe context permits, words in the above Detailed Description using thesingular or plural number may also include the plural or singular numberrespectively. The word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list. The word “exemplary” is usedexclusively herein to mean “serving as an example, instance, orillustration.” Any implementation described herein as “exemplary” is notnecessarily to be construed as preferred or advantageous over otherimplementations.

The disclosure is not intended to be limited to the implementationsshown herein. Various modifications to the implementations described inthis disclosure may be readily apparent to those skilled in the art, andthe generic principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. The teachings of the invention provided herein can beapplied to other methods and systems, and are not limited to the methodsand systems described above, and elements and acts of the variousembodiments described above can be combined to provide furtherembodiments. Accordingly, the novel methods and systems described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the disclosure. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the disclosure.

What is claimed is:
 1. A low voltage system comprising: a capacitorbetween an output node of an amplifier and ground, the output nodeconnectable to a load, the amplifier configured to operate with a seriesof pulses; a monitoring circuit configured to monitor a voltage at thecapacitor against a desired low voltage value; a control systemconfigured to generate the series of pulses for the amplifier, and tocontrol charging and discharging of the capacitor based on an output ofthe monitoring circuit to regulate the voltage at the output node atapproximately the desired low voltage value.
 2. The low voltage systemof claim 1 wherein the amplifier is an audio amplifier.
 3. The lowvoltage system of claim 2 wherein the audio amplifier is a Class D audioamplifier.
 4. The low voltage system of claim 1 wherein monitoringcircuit includes a comparator configured to compare the voltage at thecapacitor with the desired low voltage value and generate the output. 5.The low voltage system of claim 1 wherein the desired low voltage valueis a reference voltage obtained from a reference source.
 6. The lowvoltage system of claim 5 wherein the reference source is a programmablereference source configured to generate a plurality of reference valuessuch that one of the reference values is the reference voltageapproximately equal to the voltage at the capacitor.
 7. The low voltagesystem of claim 1 wherein the control system is configured to controlthe amplifier by pulse-width modulation (PWM).
 8. The low voltage systemof claim 1 wherein the output node is connected to an output stage ofthe amplifier.
 9. The low voltage system of claim 8 wherein the outputstage includes a MOSFET circuit having first and second transistorsarranged in series between a supply node and ground, such that theoutput node is at a node between the first and second transistors. 10.The low voltage system of claim 9 wherein the first transistor is a PMOSdevice and the second transistor is an NMOS device, such that a sourceof the first transistor is coupled to the supply node, a drain of thefirst transistor is coupled to a drain of the second transistor, and asource of the second transistor is coupled to the ground.
 11. A modulecomprising: a packaging substrate; and an audio amplification systemimplemented on the packaging substrate, the audio amplificationincluding an output stage operable with a series of pulses, the outputstage having an output node; and a low voltage system including acapacitor between the output node of the amplification stage and ground,the output node connectable to a load, the low voltage system furtherincluding a monitoring circuit configured to monitor a voltage at thecapacitor against a desired low voltage value, low voltage systemfurther including a control system configured to control charging anddischarging of the capacitor based on an output of the monitoringcircuit to regulate the voltage at the output node at approximately thedesired low voltage value.
 12. The module of claim 11 wherein the outputstage of the audio amplification system is implemented on a die.
 13. Themodule of claim 12 wherein the capacitor is implemented outside of thedie.
 14. A wireless device comprising: an antenna for receiving awireless signal; an audio amplification system configured to amplify aninput signal representative of the wireless signal, the audioamplification including an output stage operable with a series ofpulses, the output stage having an output node; a low voltage systemincluding a capacitor between the output node of the amplification stageand ground, the low voltage system further including a monitoringcircuit configured to monitor a voltage at the capacitor against adesired low voltage value, low voltage system further including acontrol system configured to control charging and discharging of thecapacitor based on an output of the monitoring circuit to regulate thevoltage at the output node at approximately the desired low voltagevalue; and a speaker coupled to the output node of the output stage andconfigured to generate sound waves based on the amplified signal fromthe audio amplification system.
 15. The wireless device of claim 14wherein the wireless device is a wireless headphone or a wirelessearphone.